Conventionally, there has been known an image formation apparatus using a belt-shaped member as one of its components. For example, there is one which has an intermediate transfer belt disposed opposite to a photosensitive body formed with a rotator rotatably driven, and rotated at the same peripheral speed as that of the photosensitive body, with which the belt is kept contact during the rotation. This type of image formation apparatus, which uses a method of rotating the intermediate transfer belt in one direction, has a sensor that optically detects a mark (hereafter referred to as a position detection mark) which passes under or over sensor. More specifically, the position detection mark is formed with an optically reflecting member, and it is provided on the intermediate transfer belt. This type of apparatus also comprises a charging unit, an optical writing unit, a development unit, a primary transfer unit, and a cleaning unit provided around the photosensitive body.
As a material of the intermediate transfer belt, for example, a dielectric-base organic resin film is used.
When an image is to be formed by the image formation apparatus, an image forming process is started at a predetermined timing after the detection of the position detecting mark by the detection sensor during rotation of the photosensitive body and the intermediate transfer belt. That is, optical writing to the photosensitive body is started.
When the image forming operation is finished, a drive motor which rotatably drives the intermediate transfer belt stops, thereby, the intermediate transfer belt also stops. In this case, it is programmed that the drive motor stops after a predetermined desired time since the sensor detects the position detecting mark. As a result, each time the intermediate transfer belt stops at the same position.
It is necessary that the intermediate transfer belt stops at the same position in order to keep constant a time required for outputting a first copy.
Further, in an image formation apparatus for forming color images, a toner image in a specific color is formed, by optical writing and development, on a charged area of the photosensitive body charged by passing through the charging position, and the toner image in this specific color is transferred to the intermediate transfer belt by the primary transfer unit in a primary transfer section. The image formation apparatus repeats such operations in different colors, forms a color-superimposed toner image on the intermediate transfer belt, and collectively transfers this color-superimposed toner image onto transfer paper to obtain a color image. The superimposed toner image is formed on the intermediate transfer belt with no displacement between the colors by starting the image forming process at a predetermined timing after the detection of the position detecting mark in the detection sensor in each of the different colors.
However, since the intermediate transfer belt is stretched by a plurality of holding rollers, if the intermediate transfer belt stops at the same position at any time as explained above, curl due to the holding roller may occur on a particular portion of the belt wrapped around each of the holding rollers after some time has elapsed.
When this curl occurs, a blank band may occur in an image at the time of primary transfer, which may cause the image to be failed.
To overcome that problem, Japanese Patent Application-Laid Open H06-289684 discloses an apparatus configured to have a plurality of position detecting marks on an intermediate transfer belt and allow the intermediate transfer belt to stop at a plurality of positions. In this apparatus, after cleaning of the intermediate transfer belt is finished, the sensor starts counting the number of position detecting marks having passed through a detection area, and stops the intermediate transfer belt, for example, when the sensor has counted a lower number by one than the total number of the position detecting marks printed on the intermediate transfer belt. Thus, prevention of the inconvenience may be possible.
However, in case of the apparatus disclosed in Japanese Patent Application-Laid Open H06-289684A, the cost might be increased Because a plurality of position detecting marks have to be provided on the intermediate transfer belt.
The above mentioned problem is not confined to the intermediate transfer belt, but may possibly come up in any belt-shaped member that is stretched by a plurality of holding rollers.